The Sky This Week, 2017 March 14 - 21

The Moon moves into the morning sky this week, coursing her way through the stars of late spring and early summer. Last Quarter occurs on the 20th at 11:58 am Eastern Daylight Time. Luna forms an attractive triangle with Jupiter and the bright star Spica in the pre-dawn hours of the 15th. On the morning of the 20th you’ll find the Moon two degrees above the planet Saturn.

Although it doesn’t look like it from where I sit today, spring is just around the corner. The vernal equinox occurs on the 20th at 6:29 am EDT. At this moment in time the apparent center of the Sun’s disc reaches an ecliptic longitude of zero degrees. This also happens to be the point in the sky where the ecliptic plane intersects the celestial equator, so the Sun appears to move into the northern hemisphere of the sky. At the moment of the equinox Old Sol will stand directly over the equator above a point in central Africa. From an astronomical point of view the vernal equinox is an important marker in the sky. We measure the length of the year with respect to it, and the zero-point of the sky’s "right ascension" system is anchored to it. However, the equinox itself is not fixed to the stars. Thanks to the slow wobble of the Earth’s axis known as precession, the equinox slowly moves around the celestial sphere, completing one cycle in roughly 26,000 years. On an annual basis, the equinox moves westward against the distant stars by about 50 arcseconds, roughly the apparent diameter of the planet Jupiter. In ancient times the equinox was known as the "First Point of Aries" since it was located in that Zodiacal constellation some 4000 years ago, but today it is located on the constellation of Pisces. It will move into the constellation of Aquarius around the year 2600.

There’s quite a bit of action in the evening twilight sky this week. As Venus continues her precipitous plunge toward the Sun, the fleet planet Mercury vaults upward from the western horizon. This will be Mercury’s best evening apparition for the year, and you should have several evenings to photograph the two planets together during the course of the week. The best evenings for spotting the two worlds will be the 17th through the 20th, when Venus will be about 10 degrees to the right of Mercury relative to the horizon. Venus will show an extremely thin crescent in the telescope eyepiece, while Mercury will reveal a much smaller gibbous disc. Act quickly to catch Venus as she will rapidly disappear after the 21st.

Mars is now drifting eastward through the stars of the constellation Aries. He will continue to linger in the early evening sky for several more weeks as he wends his way toward the brighter stars of Taurus. Right now he’s fairly easy to spot as an isolated reddish "star" in the western sky as twilight transforms to darkness.

The switch to Daylight Time has temporarily delayed Jupiter’s entrance into the evening sky, but by the end of the week you’ll see Old Jove crest the eastern horizon at 9:00 pm. He should be high enough in the southeastern sky by 11:00 pm to train the telescope on his cloud-festooned surface. If you’re up before the Sun on the 15th you can see him in an attractive grouping with the Moon and the bright star Spica.

Saturn may be found lurking among the rising summer constellations in the southern sky as morning twilight gathers. You’ll find him located just two degrees south of the last quarter Moon on the morning of the 20th.